1. Field of the Invention
This invention relates to a digital control system for an internal combustion engine, and more particularly to an improvement in the method and apparatus of storing and processing digital output data from an analogue to digital convertor for optimal use by a processor of the control system.
2. Description of the Prior Art
It is known to utilize a digital control system for an internal combustion engine. The digital control system generally employs sensors which provide analogue output signals indicative of engine operating conditions such as air flow rate, engine coolant temperature, power source voltage, and others. In such a control system, an analogue to digital convertor (A/D convertor) transforms the analogue signals indicative of engine operating conditions into the corresponding digial signals, and a microprocessor receives the digital signals to determine the amount of fuel to be injected into the engine and the ignition timing to control the engine in response to the engine operating conditions.
The desired degree of accuracy in the transforming process of the A/D convertor depends on the kind of analogue input data. The data indicative of the air flow rate, for example, having a relatively great dynamic range needs an accuracy higher than that of the data having a relatively small dynamic range, such as data indicative of engine coolant temperature or the power source voltage. However all such data are commonly transformed into a fixed-bit, e.g., 10-bit, digital signals for simplifying the making of the system and especially the A/D convertor. These 10-bit signals are stored in a two-stage register each stage having one byte. Such a conventional system has a characteristic that data must be read out from both the register stages, necessitating two reading operations even when low accuracy is all that is required e.g. 8-bits, as will be described hereinafter. Such operation makes the calculation process complicated and the operation time undesirably long.